Method and apparatus for receiving electrical communications



E. A. TUBBS Aug. 6, 1940.

METHOD AND APPARATUS FOR RECEIVING ELECTRICAL COMMUNICATIONS 2Sheets-Sheet 1 Filed July 30, 1937 Rm O 5 Y .E m N E R V 7 O W & m NW3g- 6, 1940. v E. A. TUBBS 2.210.738

METHOD AND APPARATUS FOR RECEIVING ELECTRICAL COMMUNICATIONS Filed Ju lyso, 1937 '2 Sheets-Sheet 2 c o l0 0 0 I 2 3 4 5 6 7 8 9 10 ll 0 l 2 3 45 G 7 8 9 l0 ll Kc INVENTOR fE/VEST A. 70555 BY M1 WM ATTORNEY PatentedAug. 6, 1940 UNETED STATES PATENT orrice METHOD AND APPARATUS FORRECEIVING ELECTRICAL COMMUNICATIONS Application July 30, 1937, SerialNo. 156,489

11 Claims.

This invention relates to communication apparatus, and especially to amethod and apparatus for receiving electrical communicationsas,

' for instance, carrier currents modulated by sound or otherintelligence.

One of the objects of the invention is to provide a receiver in whichthe fidelity of reproduction of the received signals is greatly improvedover apparatus of the prior art, and at the same '10 time interferingnoises are greatly decreased.

Another object of the invention is to provide a radio receiver for veryaccurately reproducing the signals and incorporating means to controlthe frequency band width of the received signal.

Still another object of the invention is to provide a high fidelityradio receiver with means to eliminate the beat note which may beproduced by an adjacent unwanted carrier beating with the carrier of thewanted signal.

Still another object of the invention is to provide an improved type ofautomatic volume control for a radio receiver.

Another object of the invention is to provide an improved arrangementfor a visual tuning '2 indicator for a radio receiver;

A further object of the invention is to provide a radio receiver inwhich the received signal is separated into two channels, including abroad channel and a sharp channel, and which the original signal isproduced by beating the signals in the two channels together.

Another object of the invention is to provide a radio receivingapparatus in which unequal efiects upon different frequencies of thesignal may be wholly or partially compensated for.

Other objects of the invention and objects relating particularly to thearrangement and connections of the various parts of the circuit will beapparent as the description of the invention proceeds.

One embodiment of the invention has been illustrated in the accompanyingdrawings, in which:

1 represents a circuit'diagram of a superheterodyne radio receiver forsound reproduction;

Fig. 2' represents certain phase curves and audio response curvesproduced by the two channels; and ac Fig. 3 represents an audio outputcurve for the demodulation stage ofthe set.

The invention comprises a radio receiver in which the selected signal isseparated with two channels, one of which is broad enough to pass all ofthe frequencies of the signal and the other is quite sharp. The broadchannel feeds into a balanced demodulator circuit in such a manner thatsubstantially all of the signal is normally balanced out. The signal inthe sharp circuit is, however, also fed into the demodulator cir- 5 cuitin such a manner that beating takes place between the two signals, thusproducing demodulation. Certain advantages are obtained in this mannerwhich will be subsequently explained. A device for controlling thefrequency band width of the broad channel is also a feature of theinvention; and automatic volume control and visual tuning indication arearranged in a novel manner to produce advantageous results.

In the circuit diagram of Fig. 1 a unit It] of conventional design mayinclude the tuning circuit for selecting the desired incoming signal, anoscillator for producing the auxiliary frequency to beat with theincoming signal and thus form the intermediate frequency, and a mixertube where this beating action takes place. An antenna II may beconnected to the unit II] for picking up the signal and the unit III maybe grounded at I2, as indicated. The output circuit from the unit II],and thus from the mixer tube therein, may contain the coil I3 which maybe shunted by the condenser I4, forming a resonant circuit which istuned to the intermediate frequency. The plate of the mixer tube may besupplied with a high positive potential from a source to be laterdescribed, through a resistance I5, which may be connected to the coilI3. The lower end of the coil I3 may be connected to ground through aby-pass condenser I6.

The coil I3 may form the primary coil of a g transformer II, thesecondary I8 of which may be shunted by a variable condenser I9 to tuneit to the intermediate frequency, and may have one end connected to thegrid 20 of an amplifier tube 2 I, while the other end may be connectedthrough a resistance 22 to a suitable source of potential, in thepresent instance shown associated with the automatic volume control, tobe described hereinafter. The lower end of the coil I8 may also beby-passed to ground through a condenser 23.

For reasons to be hereinafter explained, I prefer to keep the plate togrid capacity as low as possible in the amplifier tube 2|, and hence Iprefer to use a pentagrid tube having an anode 24, a suppressor grid 25,a second control grid '50 26, a screen grid 21 around the second controlgrid 26, and a cathode 29 which may be independently heated by asuitable heating filament, not shown but well understood in the art. Inusing this type as an amplifier I prefer'to conmay connect the screengrid to ground, while the cathode 29 may be connected to ground througha resistor 32 which may be by-passed by a condenser 33.

The plate 24 of the tube 2| may be connected to the primary coil 34 of atransformer 35 which couples the amplifier tube 2| to a second amplifiertube 36. The coil 34 may be tuned by a variable condenser 31 to theintermediate frequency, this condenser being in series with a fixedauxiliary condenser 38 which is used for a purpose to be laterdescribed. The lower end of the coil 34 may be given a suitable highpotential from a source to be hereinafter described, through aresistance 39 with the end of the coil bypassed to ground by means of acondenser 40.

The transformer 35 may have a secondary winding 4| which may be shuntedby a variable condenser 42 in order to tune the coil to the intermediatefrequency, and one end of the coil may be connected to the control grid53 of the tube 36. This tube may be a pentagrid tube with its elementsconnected exactly the same as those of the tube 2|, and these individualelements need not be described in detail. The plate 44 of the tube maybe connected to one end of a coil 45 which forms the primary winding ofa transformer 46. This coil may also be tuned. to the intermediatefrequency by a variable condenser 41 shunted across it and in serieswith a fixed condenser 48, the latter being for a purpose to be laterdescribed. The lower end of the coil 45 may be given a high positivepotential from a source to be later described, through a resistance 49,while a by-pass condenser 5|] may be connected between the lower end ofthe coil and ground.

The transformer 46 may have a secondary winding 5| which forms the inputcircuit for a balanced demodulator comprising two tubes 52 and 53,connected in push-pull, one end of the winding 5| being connected to thesecond control grid 54 of the tube 52, and the other end to the secondcontrol grid 55 of the tube 53. A variable condenser 56 may be connectedacross the coil 5| to tune it to the intermediate frequency, The coil 5|may have a center tap to which the wire 51 may be connected which leadsto a source of potential for the second control grids of the tubes 52and 53, and which will be later described. This center tap may beconnected to ground through a condenser 58.

The plates 59 and 60 of the tubes 52 and 53 may be connected to oppositeends of a coil 6| which forms the primary of an iron core transformer52. A tap at the midpoint of the primary 6| may be given a positivepotential in a manner to be hereinafter described. The tubes 52 and 53may be similar, if desired, to the tubes 2| and 36, already described,having, in addition to the second control grid and plate, mentionedabove, suppressor grids, cathodes and inner control grids 33 and 64,which may be connected together and which form the means of combiningthe sharp channel with the broad channel.

Suitable screen grids are provided between the inner and outer controlgrids to shield these grids from each other. However, I have found thatin most instances this shielding is not sufficient to completely preventone grid from affecting the nect the suppressor grid 25 and the secondconother, and I therefore provide a condenser 65 having two separatestators, one of which may be connected to the control grid 54 and theother to the control grid 55, and a single rotor which may be connectedto both the control grids B3 and 64. By adjusting this condenser I amable to equalize the circuits including the two halves of the coil 5|.The cathode, screen grid, and suppressor grid connections for the tubesmay be similar to those described in connection with the tubes 2| and36.

With the arrangement just described, a signal picked up on the antennaand selected by the tuner in the unit I0, may be converted to theintermediate frequency by the mixer tube and amplified by the tubes 2|and 35. The combined effect of all of the circuits preceding the tubes52 and 53 may be such as to receive a wide channel of frequencies, whichwill include all of the side bands to produce the complete audio signal,and these circuits may be similar to standard circuits incorporated witha superheterodyne receiver, with the exception that they may be somewhatbroader, because in the average superheterodyne receiver the circuitsmust be made narrow enough to eliminate any interference from unwantedstations, while with the present invention I may make these circuitsbroad enough to insure the complete reproduction of the wanted signal,and I need not be greatly concerned with any interference which may comein adjacent the outer side bands of the wanted signal.

The intermediate frequency with the signal impressed thereon isintroduced to the tubes 52 and 53, which form the balanced demodulator,by means of the second control grids 54 and 55 respectively, of thesetubes. It will be noted that these grids are connected in push-pull,and, excluding the effect of the other control grids, if

the circuits are properly balanced and the characteristics of the tubesare the same, the tubes will act to produce amplification of theintermediate frequencies applied to the grids without any detectoraction. This is because any detector action in one tube will be balancedout in the primary 5| of the output transformer 52 by the same detectoraction in the other tube. However, if a frequency corresponding to thecarrier frequency is applied to the two control grids 63 and 54 of thetubes 52 and 53, and this frequency is something other than degrees outof phase with the carrier frequency in the broad channel, (preferably inphase with it or degrees out of phase) beating will take place in thetubes 52 and 53 between this frequency and the side bands included inthe broad channel to produce detector action and cause signal currentsto flow in the primary 6|,

In order to obtain this additional frequency, which corresponds to thecarrier frequency, I may use another oscillator with means to keep it instep with the carrier frequency, but I may prefer to take some of theincoming signal and filter out or otherwise remove a considerableportion of the outer side bands and apply the remaining frequencies tothe first control grids of the tubes. In the drawing, I have shown onearrangement for carrying out the latter procedure. A tube 66 may haveits control grid 61 connected to the control grid 43 of the tube 35,thus connecting the control grids of the tubes 66 and 36 in parallel. Bythis means the modulated intermediate frequency in the output circuit ofthe tube 2| is impressed equally on the grids of tubes 35 and 66. Thetube 66 may be a pentagrid tube with the suppressor grid 68 and a secondcontrol grid 69 connected together and to the cathode 19. which may beconnected to ground through a re sistance H shunted by a condenser E2.The screen grids I3 may be given suitable potential through a resistanceI4 from a source to be later described, and may be also connected toground through a condenser 15,

The plate I6 of the tube 66 may be connected to one end of a coil 11,which may be shunted by a variable condenser I8, and the other end ofwhich may be connected through a resistance 19 to a source of potentialto be hereinafter described. The lower end of the coil Il may beconnected to ground through a condenser 89.

In order to narrow the band width of the sharp channel and remove thedesired amount of outer side bands and secure a suitable phase rotationbefore these frequencies are introduced to the control grids 63 and 64of the tubes 52 and 53, I may couple three resonant circuits BI, 62, and93 in cascade to the coil TI. The resonant circuit BI may comprise acoil 64, which may be shunted by a variable condenser 95 and anothercondenser 86 in series with it, while the lower end of the coil may beconnected to ground as shown.

The resonant circuit 82 may comprise a coil 81 which may be shunted by avariable condenser 86 and another condenser 89 in series with it, thelower end of the coil being connected to ground. The juncture of thevariable condenser 85 and the other condenser 86 may be connected bymeans of the wire 99 to the juncture of the condenser 88 and condenser89, thus coupling the resonant circuit 82 to the resonant circuit 8!.

The resonant circuit 63 may comprise a coil 9! which may be shunted by avariable condenser 92, one end of this coil being connected to the grids63 and 64 of the tubes 52 and 53 respectively. The other end of thiscoil may be given a suitable negative potential in a manner to bedescribed. The coil 9 I' may be inductively coupled to the coil 8'I,there being provided a core 93 between the two coils, which ispreferably made of small particles of magnetic material insulated fromeach other, so that the efficiency of energy transfer may be highwithout substantial attenuation of the frequencies to be carried.

By the use of these three resonant circuits 8!, 82 and 83, coupled tothe resonant circuit containing the coil 11, all of which are preferablyloosely coupled together, I succeed in considerably reducing the bandwidth of the signal in this sharp channel, so that the carrier frequencyapplied to the grids 63 and 64 of the tubes 52 and 53 may be without anysidebands at all, or at least accompanied by limited side bands, theouter ones of which are greatly attenuated. I am also able by means ofthese coupled circuits to obtain the desired phase rotation of thefrequencies in the sharp channel so that maximum detection may beobtained.

The beating action between the sharp signal applied to the control grids63 and 64 and the broad signal applied to the second control grids 54and 55 produces, as has already been stated, an audio signal in theprimary 6| of the transformer 62. This signal may be amplified by anysuitable audio amplifier and reproduced by any suitable translatingdevice. To this end the secondary 94 of the transformer 62' may beshunted by a resistance 95, having a movable contact which may beconnected to the control grid 96 of an amplifier tube 91. One end of thecoil 94 may be grounded, as indicated, and the resistance may be tappeda short distance from the grounded end, and this tap may be connectedthrough a condenser 98 and a variable resistor 99 to ground. The movablecontact on the resistance 95 may be adjusted to control the volume ofthe set, while the tone may be controlled by variation of the resistance99.

The suppressor grid I90 of, the tube 91 may be connected to the cathodeI9I, which may be connected through a resistance I02 to ground, theresistance being shunted by a condenser I93. The screen grid I04 of thistube may be given a suitable potential through a resistance I95, theends of which may be connected to ground through condensers I96 and I91.The manner of applying the potential will be hereinafter described.

The plate I98 of the tube 91 may be connected through a tuned trap I09and a resistance '9 to a source of positive potential to be hereinafterdescribed. A condenser I I I may connect the juncture of the resistanceH9 and the tuned trap I09 to the control grid H2 of an amplifier tube 3,this grid being provided with a high resistance leak II4 to ground. Thistube may be similar to the tube 91, having a suppressor grid H5 whichmay be connected to the cathode I I6, the later being connected, toground. through a resistance II'I shunted by a condenser H8.

I may connect the screen grid H9 and the plate I29 together and througha primary winding I'2I of a transformer I22 to the source of I platepotential. The secondary winding I23 of the transformer I22 may have itsends connected to the grids I24 and of the power tubes I26 and I21,respectively, which are thus connected in push-pull. The filamentarycathodes I28 and I29 of these tubes may be connected in parallel and toa source of filament potential not shown. The mid-point of the secondaryI23 may be connected to a source of grid biasing potential, to behereinafter described, and the plates I39 and I3I of the tubes I26 andI2! may be connected to the ends of a primary winding I32 of atransformer I33, the mid-point of the primary I32 being connected to thesource of positive potential. The secondary I34 for the transformer I33may be connected to the loud speaker I35 or other translating device.

A suitable power supply I36 may be provided for the circuit which may beconnected by means of the cable I 31 to the source of householdalternating or direct current, as will be well understood. This powersupply may be of any well known type for providing a plurality ofdifferent positive potentials for the plate and screen grid circuits ofthe various tubes, as well as negative potentials for biasing thecontrol grids. Thus a high potential terminal I38 on the power supplymay be connected to the mid-point of the primary I32 of the loud speakertransformer I33 and through a resistance I39 to the resistors H9 and I05in the plate circuit and screen grid circuit of the tube 91.

A lower potential terminal I49 may be connected to the primary coil I'2Iin the plate circuit of the tube H3, to the primary coil 6| in the platecircuits of tubes 52 and 53, to the resistance 49 in the plate circuitof the tube 36, to the resistance 39 in the plate circuit of the tube2!, to the resistance I5 in the plate circuit of the mixer tube in theunit I9, and to the resi'stance '19 in the plate circuit of the tube 66.Also the terminal M0 may be connected to a resistance MI in the screengrid circuit of the tubes 52 and 53, and through a resistance I42 tolower the potential, and then through resistance Ell in the screen gridcircuit of the tube 2! and through resistance M in the screen gridcircuit of tube 66.

Grid potentials for the various tubes may be provided by a groundedresistance I43 which may be connected to the negative terminal M4 of thepower supply unit. This terminal may also be connected to ground througha condenser M5. The midpoint of the secondary I23 of the transformer I22may be given a suitable potential for the grids Q24 and I25 of the tubesI26 and E21 b connecting it directly to the terminal EM. A point I46 onthe resistance M3 may be connected to the mid-point of the coil 5| togive a suitable potential to the second control grids 5 and tI- of thetubes 52 and 53, and this point is preferably made adjustable tofacilitate obtaining the proper grid bias after the grid circuits havebeen balanced.

In order to provide a sensitivity control for the receiver, I mayprovide a resistance 5620; which may be connected between ground and thescreen grid potential supply resistance i i-ii, so that current alwaysflows through the resistance. A movable tap M3a on this resistance maybe connected to the cathodes of the radio frequency amplifier tubes inthe unit It, to thus control the bias on these tubes and therefore thegain thereof.

With the arrangement so far described, a broad band of signalfrequencies, picked up by the antenna l and suitably selected by theunit ill and converted into intermediate frequencies, is amplified bythe amplifier ill and then divided into two channels, the tube 36amplifying the broad channel and transferring this broad band offrequencies to the balanced demodulator tubes 52 and 53. The band offrequencies of the other channel is amplified by the tube 66 and thennarrowed down by the resonant circuit of the coil 1'! and the threeresonant circuits 3!, 82 and 83, which are coupled to it. The resultantnarrow band of frequencies is then applied to the other control gridsand M of the tubes 52 and 53. The beating action takes place in thesetubes and the audio signal flows through the coil 6! and is thereuponamplified by the tubes 5?, i it and E26 and I21, and translated by theloud speaker I35 into sound.

I have referred above to the broad channel and the sharp channel and thefrequency band Widths thereof. While it is generally sufficient inconsidering these two channels to compare the resonance curves thereof,the measurements of the frequency band widths are made at the usefuloperating levels. Wherever the expression frequency band width is usedin this specification or claims, therefore, it is intended to mean theWidth of the resonance curve at some predetermined operating level.

Any suitable method may be employed to narrow the band of frequencies inthe sharp circuit. I may, in some instances, use a crystal to eiiminateall of the side band frequencies and leave just the carrier alone.However, I have discovered that if the sharp channel, the voltage ofwhich is applied to the control grids tit and G l of the tubes 5?. and53, is made too sharp, it is not only difficult to tune the receiver,but extremely difficult to maintain the receiver in tune.

This is because a slight drifting of the resonant point of the sharpcircuit will reduce the signal voltage to such a point where its effecton the tubes 52 and 53 will be very small. This is especially true wherea crystal is used in the sharp circuit to filter out all of the sidebands and leave the carrier alone. A slight drift in the resonant pointof the crystal will change the strength of the voltage of the sharpcircuit from its maximum to substantially zero, and thus cause thesignal in the loud speaker to disappear entirely.

This sharp selectivity may also give rise to a motorboating action orlow frequency howl, caused by the mechanical vibrations from the loudspeaker affecting the oscillator tuning condenser, thus changing thetuning so as to move the carrier on and off of the resonant point of thecrystal.

When a crystal is used in this sharp circuit or the equivalent sharpnessof the circuit is produced, it is difficult to locate a station intuning because the operator may pass directly through the stationWithout hearing it at all, unless he is moving the tuning apparatus atan exceedingly slow speed.

In view of these disadvantages of too sharp a circuit, I may prefer togive the sharp channel a sufficient band width so that slight driftingof the resonance point of this channel will not cause the defectsmentioned. proper width of this channel I may prefer to gauge it by itseffect on the audio output curve of the set.

The audio output of the balanced demodulator may be considered to bemade up of two components: one produced by the carrier introduced by thesharp circuit beating with the side bands included in the broad circuit,and one produced by the carrier in the broad circuit beating with theside bands included in the sharp circuit. The amplitude of a givenfrequency in each component will depend on the amplitude of the sidebands in that component and the phase relation between the side bandsand the carrier with which they are beating. the vector sum of these twocomponents' It is well known that the phase of the current in a resonantcircuit rotates as the frequency of the current is shifted through theresonant point of the circuit. the rotation of the phase againstfrequency is never more than degrees. If the circuit is a complex onehaving more than one resonant circuit, the rotation of phase is morerapid, the

rapidity increasing with the number of resonant circuits used, and asthe coupling between the circuits is decreased. Thus where severalresonant circuits are used coupled together, the phase may make severalcomplete rotations as the frequency is shifted from one side of thecombined resonance curve to the other.

Thus a plurality of frequencies, including the wanted carrier and itsassociated side bands and perhaps an unwanted carrier with itsassociated side bands, passing through a given circuit, will Indetermining the The total audio output then If the circuit is a singlecircuit a.

of the broadchannel is represented by the curve A which is made byplotting kilocycles for the abscissa and angle of phase rotation for theordinates. The scale of phase rotationis shown at the right. The phasecurve for the sharp channel is represented by the curve B. Both curveshave been plotted for the audio phase rotation from zero to 11kilocycles. These curves will correspond to the phase rotation curves ofthe side band frequencies plotted on one side of the res onance point,which may then be taken as zero frequency, and the scale of kilocycleswill then represent the number of kilocycles from resonance.

From an inspection of these curves it will be evident that the audiophase produced by the broad channel component rotates rather slowly sothat a 90 degree rotation is first reached at a frequency slightly under10 kilocycles. The phase of the component of the sharp channel, with itsplurality of coupled tuned circuits, however, reaches 90 degrees at alittle under Zkilocycles, 180 degrees at about.3.5 kilocycles, and 270degrees at a little under. 7 kilocycles.

If any audio note in the broad channel component is 180 degrees out ofphase from the same note in the sharp channel component, one will cancelthe otherprovided the amplitudes are the same. If the amplitudes are notthe same the result will be the difference between the amplitudes. Ifthese audio notes have the same phase, they will add together. For allother phase differences the effect will be the vector sumof these twocomponents.

InFig. 2 I have also shown the audio amplitude curves for the componentsalready referred to of the broad and sharp channels, the curves C and Drepresenting respectively the audio amplitude curves for these channels.The former is sub-' stantially fiat to beyond 10 kilocycles and thelatter falls off rapidly in amplitude beyondabout 1.5 kilocycles. I

It willbe seen from an inspection of the. phase curves, that, as thefrequency increases from zero, the phase difference between the audionotes of the two audio components increases rapidly until at about 2kilocycles it reaches 90 degrees, while at about 3.8 kilocycles itreaches 180 degrees, and at about '7 kilocycles it has approached 240degrees, whereupon it falls again sov that at. 11 kilocycles it is againin the neighborhood of 180 degrees. As the audio frequency increasesfrom zero these two components are at first adding;

and then as their phase difference increases they finally begin tosubtract until at 180 degrees the audio produced is simply the result,of arithmetically subtracting one. component from the other.

' Taking into account the varying amplitudes of the two circuits as wellas the phase differences, the audio amplitude curve in the output of thebalanced demodulator appears as indicated in Fig. 3. By various methods,as, forexample, by means of a suitable. by-pass condenser in thecathode. circuit of the audio amplifier tubes, the low frequency peak ofthe curve may be brought down in a manner well understood phase curvefor both circuits, certain sound frequencies may be increased ordecreased, and in this way I may not only compensate for the tendency ofthe speaker or audio amplifier to reproduce or amplify certain soundfrequencies more than others, but I may create an output curve having acertain desired shape.

Under certain conditions I may arrange the phase relation between thecircuits so that the irequency at which the modulation cancels outcorresponds to the frequency of an unwanted carrier. If one of these oddmultiples of 90 degree phase difference points he made to fall atkilocycles above resonance, then there will be another such pointsubstantially 10 kilocycles below resonance, thus assuring an addedprotection against unwanted interference for the pres ent day standardof separation. I

I prefer to use automatic volume control with the receiver and themanner of applying such control is one feature of the invention. Iobtain a control from a circuit of the sharp channel which, because itis considerably sharper than the broad channel, is free from much of theinterference which is found on the broad channel. Therefore the signalmay be maintained at substantially constantvolume without being greatlyinfluenced by the interference from adjacent stations. I prefer to takethe voltage for theautomatic control from the resonant circuit 8i, andto do this I connect the juncture of the coil 34 and condenser 85through a condenser M7 to the anodes M8 of a rectifier tube M9, thecathodes I58 of which are grounded.

These anodes may also be connected through a series of resistances IBI,I52 and 53 to ground, and I then connect the grids of the various tubesto points along these resistances. A wire 554 leading from the grids ofthe radio frequency amplifier tubes in the unit ill may be connectedthrough-a resistance I55 to the juncture of the resistors 252 and IE3,this wire being also connected to ground through a condenser 856. Thegrid circuits of the tubes 2i and 36 and 56 may be connected by meansofa wire I57 through a resistance 58 to the juncture of the resistors iiiand I52.

In this manner the drop in potential across the resistances I52 and H53,as controlled by the voltage in the resonance circuit 8 i, willdetermine the'potentials of the grids in the unit it and the amplifyingtubes 2|, 36 and v$56. As the strength of the signal, therefore, risesin the sharp channel, including the resonant circuit 8|, the bias on thegrids of the various amplifier tubes increases, thereby tending toreduce the strength of the signal, and when the signal tends to fade,the bias on these tubes is automatically changed to increase the gain,and therefore increase the signal. I

It will be evident that interfering frequencies found perhaps in thebroad channel, may not be found in the resonant circuit 8! owing to theloose coupling between it and the coil "H, andtherefore suchfrequencieswill have no effect at all on the automatic volume controlwhich will be governed more nearly by the strength of the wanted signalalone.

Where the greatest fidelity is desired in a radio program it isnecessary to receive a channel which is suficiently broad to contain allof the side band frequencies of a Wanted station with a minimum ofattenuation. To accomplish this it may be'necessary to have the circuitso broad that an adjacent station separated, for instance,

by only 10 k. c. in the ether spectrum, may be picked up, either inwhole or in part. If this station has a signal strength very muchgreater than the wanted signal, the tubes may become overloaded, andhence it may be desirable to provide some means to change theselectivity of the set so as to eliminate the eifect of such a station.

In my application, Serial No. 151,805, fled July 3, 1937, and entitledMethod and apparatus for controlling frequency band width of coupledcircuits, I have shown and described a method and apparatus forcontrolling the band width of a receiver in such a manner that asufficiently wide band may be transmitted for receiving all of a wantedsignal, and this band may then be narrowed at will in order to eliminateinterference which might come in at one side of the particular stationto which the set is tuned. I have incorporated this arrangement in thereceiver shown in Fig. 1.

As explained in the application, I prefer to couple an auxiliary circuitto the primary of a transformer and to vary the band width of thetransformer by varying the Q (the reactance of the coil divided by thetotal resistance of the circuit) of the auxiliary circuit or thecoupling of that circuit to the primary coil. Under the proper valuesfor the components of the circuits I may broaden the resonance curve ofthe transformer into a substantially fiat-topped fairly wide curve, or Imay use two or more transformers in cascade and broaden each in such amanner that the combined curve is substantially flattopped with steepsides.

In the present invention I have shown three transformers i1, 35 and 4B,for the intermediate frequencies connected by the tubes-ZI and 36, and Iprefer to control the band width of the transformers 35 and 36 bycoupling auxiliary circuits to them. Thus the auxiliary tuned circuitI59, comprising the coil I60 connected in series with the variablecondenser I6I, auxiliary condenser I62, and the switch I63, may becoupled to the primary 3 3 of the transformer 35 by means of a coaxialcable I64 which may be connected between the juncture of the condensersI62 and NH and the juncture of condensers 31 and 38. The condensers 38and I62, therefore, act as coupling condensers for the cable. The cablemay have a suitable shield I65 which may be grounded as indicated.

The switch E63 may be provided with a plurality of contacts I66, I61,I68, I69 and I10. Of these five contacts I use only four, the first one,I66, on the left not being connected to anything. The last two contacts,I69 and I10 may be connected together and to the other side of thecondenser I62, while between the connected contacts I 61 and I68 andthis same point on the circuit, I may insert a resistance I1I. In thepresent instance the Q of the coil is preferably made substantially thesame as the Q of the secondary M of the transformer 35, and theresistance I61 is made a predetermined value, such that when the switchI63 is on either of the contacts I61 or I68 the Q of the circuit I59will be changed sufiiciently so that the resonance curve of thetransformer 35 will be slightly broadened. When the switch is on thecontact i66, which is open, the auxiliary circuit I59 will not affectthe primary 34, and hence the resonance curve of the transformer 35 willbe in its sharpest form. When the switch I63 is on either of thecontacts I69 or I18, the auxiliary circuit I59 has its maximum effect onthe primary-34, and the transformer 35 therefore has its broadest formof resonance curve.

In a similar manner I connect an auxiliary circuit I12 to the primary 45of the transformer 46. The circuit I12 may comprise a coil I13 connectedin series with a variable condenser I14, an auxiliary condenser I15, anda switch I16, the latter having five contacts I11, I18, I19, ISIlandI8I-. The contacts I13, I and NH may be connected together and to theother side of the condenser I15, which may also be grounded. The contactI18 may be connectedthrough resistance I82 to the other side of thecondenser I15. The circuit may be connected by means of a coaxial cableI83 between the juncture of condensers I14 and I15 in theauxiliarycircuit and condensers 41 and 48 in the circuit of the primary 45, thususing the condensers 48 and I15 as-coupling condensers. This coaxialcable may be provided with a'suitable shield I84 which may be grounded,as indicated.

-With this arrangement of the auxiliarycircuit I12, when the switch ison the contact I11 the auxiliary circuit is open and there issubstantially no effect upon the resonance curve of the-transformer 46.Upon movement of the switch to the contact I18, the circuit I12 isclosed through the resistance I82 with the result that the transformer46 has its resonance curve slightly broadened. When the switch is on anyof the contacts I19, I80 and I8I, the maximum effect of the auxiliarycircuit is produced on the transformer 46, which gives the widestresonance curve.

The combined effect of broadening the transe formers 35 and 46 is toproduce a saddle-top curve, the valley of which may then be filled in bythe single peak resonance curve of the transformer I1, which isunaltered. While any two of the three transformers I1, 35 and 46 may beacted upon to broaden the resonance curve, or all three of them may beaffected, if desired, I preferably choose the last two transformers sothat I will have the maximum number of sharply tuned circuits in thesharp channel .at all times.

It will be noted that in moving the switches from the right handposition towards the left, resistance is inserted in the circuit I59before any resistance is inserted in the circuit I12; hence the bandwidth of the transformer 35 is narrowed before that of the transformer46. Inasmuch as the sharp circuit includes the transformer 35, the firstnarrowing step also narrows the sharp circuit. This is a very desirablefeature of the invention, as narrowing the sharp circuit produces amarked increase in the selectivity of the receiver. Whenever the degreesof expansion of the expanded circuits are unequal I may prefer to keepthe expanded coupled circuit which is in the sharp channel sharper thanthe one which is only in the broad channel.

I have found that most standard tubes do not have low enough plate togrid capacity to prevent interaction between their plate and gridcircuits, and therefore I prefer to use pentagrid tubes for the tubes2|, 66 and 36 and to connect them as indicated. This is particularlyimportant where an overall broad fiat top curve is desired. I have foundthese pentagrid tubes to give fairly good results, although still theoperation is not perfect.

Great care should preferably be used in shielding the auxiliary circuitsI59 and I12 from each other and from the rest of the circuit so as toprevent feedback, which may be. extremely. detriat all times.

7 the tube 9?.

mental to the operation of the receiver. To avoid confusion thisshielding has not been shown.

When the resonance curves of the transformers 35 and IS are broadenedthere is attenuation in the circuit so that a lower signal tends to beproduced in the output. The attenuation in the transformer 35 may becompensated for by the automatic volume control, but the transformer 49does not affect the automatic volume control. Therefore the action ofthis transformer has the disadvantage that in tuning the set, if anadjustment is made to broaden the circuits and thus increase thefidelity of the received station, the volume will tend to decrease. Inorder to prevent this I provide a plurality of resistances I95, I86, I81and I88, which may be connected in series between ground and the lowside of the resistance I 42 in the screen grid supply circuit, so thatcurrent flows through these resistances I then ground the cathode I89 ofthe tube 35 through a resistance I99 and some portion of the series ofresistances I85 to I88, and I prefer to do this by means of a switch I9|which may sweep across a plurality of contacts I92, I93, I94, I and I99.The contact I92 may be connected to the juncture of the resistances I95and E86. The contact I99 may be connected to the juncture of theresistance I86 and Gill. The contact I99 may be connected to thejuncture of the resistances I81 and I88, and the two contacts I95 andI96 may be connected together and to ground.

Inasmuch as considerable current is flowing through the resistances I85to I88 inclusive, the movement of the switch I9I to connect points onthese resistances to the cathode I89 will vary the potential on thecathode and therefore vary the gain of the tube 36. Thus, by operatingthe switch I9I at the same time that the other switches for controllingthe band width are operthe particular type of curve desired, and thearrangement shown merely illustrates one manner in which the desiredresult may be obtained.

As has already been mentioned, I preferably provide a tuned trap I99 inthe plate circuit of This trap is preferably chosen so that it willabsorb a ten thousand cycle note. When the set is operating with itsbroadest resonance curve in the broad channel to receivewith maximumfidelity the signal to which it is tuned,

. it may then be broad enough to receive a carrier of the next adjacentstation which maybe spaced at 10 k. c. from the carrier of the receivedsignal. The two carriers, the wanted and the unwanted, may then beattogether producing a 10 k. c. note which is reproduced by the loudspeaker as a high whistle and which may be objectionable to many people.Hence, the trap I99 may be used to filter out this 10 k. c. note. I havefound that with its use-although the ob jectionable note is removed,very little difference is noticed in the quality of the wanted signal.

I prefer to have the trap in the circuit at all times but to providemeans to short it out when the broad channel is adjusted to have itsbroadest resonance curve. For this purpose I may provide a switch I91which may have five contacts I98, the firstfour of. which may be leftwithout connection, and the last one of which may be connected to oneside of the trap I09, while the switch arm I9'I is connected to theother side. With the switch on one of the first four contacts the trapis in the circuit, while when the switch is on its lastccntact, the trapis short-circuited.

The trap is made to absorb a 10 k. 0. note because It) hilocycles hasbeen adopted as the standard distance in the ether spectrum forseparating broadcasting stations throughout the United States. Thecharacteristics of the trap may of course he changed to comply with anyother separation standard, as will be understood.

As it is convenient to operate the band width changing device, as wellas the 10 k. c. trap, with a single control, I prefer to connect thearms of the switches I63, IIII, I'IE and I91 all on the same shaft, asindicated by the dotted line I99, to which a single knob (not shown) maybe applied so that all four switches may be rotated at the same time.Therefore, when all the switches are on. their farthest left handcontacts the band width controlling circuits are completely open so thatthe sharpest resonance curve is produced. Also the cathode biasingswitch I9I has given the cathode of the tube 99 a predetermined bias andthe I639 is connected in the plate circuit of the tube 97. When theswitches are moved towards the right to their next position, the bandwidth of the broad channel becomes slightly broader, while at the sametime a slightly different biasis given for the cathode of the tube 98 toincrease the gain of that tube. Connection forthe tunedtrap I99 remainsthe same. Movement of the switches to the next contact still furtherbroadens the circuit, increases the gain of the tube iifi, but does notchange the tuned trap Hi9 which is still in the circuit. If desired,then, the operator may move the switches to the last contact to theright, and inasmuch as the last two contacts are connected together inthe switches III-Ii, I9! and lit, there will be no change in the bandwidth or the gain, but the tuned trap I09 will be short-circuited. Theselast two contacts, therefore, merely operate to short-circuit the tunedtrap I99 or cut it into the circuit so that if the operator has theswitches in their farthest position to the right, and is disturbed by a10 k. 0. note, he can turn the switches back one position and eliminatethe note.

When the phase relation between the carrier in the sharp channel and thesame carrier in the broad channel is equal to zero or degrees, the bestphase relation for maximum demodulation obtains. When the phasedifference is 90 degrees or some odd multiple of 90 degrees, theconditions are the least favorable for demodulation. Therefore when thebroad and sharp channels are composed of a different number of tunedcircuits it will be seen that in the process of tuning in a station itmay be possible to alternately go through several phase points which mayproduce maximum and minimum demodulation. The operator will then passthrough several points adjacent the resonant point where the signal willalternately reach a maximum and a minimum, and he therefore may havedifliculty in selecting the one which corresponds to the resonancepoint.

If the operator stops on any point other than the resonance point thequality of the reproduced signal may be considerably impaired. There aretwo principal reasons for this: One is that as the carrier is nearer oneside of the resonance 75,

curve than the other the uniformity of the side band amplitudes isupset, some of the side bands, perhaps, being out 01f completely on oneside, and the phase relation of the side bands is altered, so that theoverall audio amplitude curve may take an undesirable shape. The otheris that interference from one side may be introduced.

For this reason a visual tuning indicator is highly desirable. Thearrangement for connecting such an indicator to be operated on the sharpchannel is a feature of the invention. Forthis purpose I may use one ofthe visual indicating devices now in general use, such as the No. 6G5tube. In order to operate this device I may connect the coil 9| of theresonant circuit 83 in the sharp channel to the grid 200. of a tube 2!,which I prefer to connect between the resonant circuit 23 and rectifierfor the visual device to prevent damping of the resonant circuit. Thetube 2|]! may be of the combined pentode and double diode type, and theplate 202 may be connected through a resistance 203 to the source ofpositive potential on the lower terminal I of the power supply. Theplate 202 may also be connected through a condenser 204 to the diodes205 in the same tube, and these diodes may also be connected through aresistance 206 to the cathode 20'! of the tube. The diodes may also beconnected through a resistance 208 to the grid 209 of the visual tubeZIO, which, as has already been mentioned, may be of the 6G5 type. Thegrid 209 may also be connected through a condenser 2 to ground, asindicated, the resistor 208 and condenser 2H forming a filter circuit toprevent radio frequencies from reaching the grid 209.

Variation of voltage on the grid 209, as produced by the direct currentdrop in potential across the resistor 206, will vary the visual effecton the target 2l2 of the tube Zlll in accordance with well knownprinciples, and the operator can readily see when the set is tuneddirectly on the carrier of the desired station. Inasmuch as the visualindicating device is connected to the resonant circuit 83 which is theoutput of the sharp channel, the circuit may be tuned much moreaocurately than if the visual indicating device were in the broadlytuned circuit.

From the above description it will be evident that I have provided areceiver for a modulated carrier wave which will reproduce the signal ata substantially constant volume with a high degree of fidelity and witha minimum of interference. The tuning is extremely accurate and anadjustment may be made to eliminate interference at a slight sacrificeof quality, if necessary.

It will be noted that the circuit is designed to prevent overloading sothat the tubes in the sharp circuit are operated below their saturationpoint. I have found that if these tubes become overloaded distortion maybe produced by the beating of the carrier in the broad channel withdistorted side bands in the sharp channel. Under this condition also thetuning is more difiicult as there appears to be a considerable decreasein volume exactly on the resonant point from points closely positionedat either side thereof.

Many modifications may be made in the circuit connection and the numberand character of the parts used without departing from the spirit of theinvention, and I do not, therefore, desire to be limited except by thelimitations included in the appended claims.

What I claim is:

1. An apparatus of the class described comprising, means to intercept acarrier wave and its associated side bands, means to divide the signalthus intercepted into two channels, one of said channels being narrowerin frequency response than the other, means to substantially eliminatedemodulation in the broad channel, means to cause the frequencies in thetwo channels to beat together means responsive to the beat frequenciesproduced by beating the oscillations of the two channels fordemodulating the said beat frequencies to produce only the desiredsignal, and means controlled by said narrow channel to maintain thevolume of the demodulated signal at a substantially constant level.

2. An apparatus of the class described comprising means to intercept acarrier wave and its associated side bands, means to divide the energythus intercepted into two channels, means to reduce the band width offrequencies in one of said channels without eliminating all of said sideband frequencies, means to substantially eliminate demodulation in theother of said channels, means to cause the frequencies in the twochannels to beat together to produce demodulation, and means fordemodulating the beat frequencies so produced by beating theoscillations of the two channels to produce only the desired signal.

3. An apparatus of the class described comprising means to intercept acarrier wave and its associated side bands, means to divide the energythus intercepted into two channels, means to reduce the frequency bandwidth of one of said channels, means operated by said last mentionedchannel to maintain substantially constant the gain of both saidchannels, and means to beat the frequencies of said channels againsteach other, and means for demodulating the beat frequencies so producedto produce only the desired signal.

4. An apparatus of the class described comprising means to intercept acarrier wave and its associated side bands, means to divide the energythus intercepted into two channels, means to reduce the frequency bandwidth in one channel, means controlled by said last mentioned channel toautomatically vary the amplitude of the intercepted carrier wave andassociated side bands, and a visual indicator for tuning the apparatusoperated from said last mentioned channel at a point therein having anarrower band width than the point at which the amplitude is controlled.

5. An apparatus of the class described comprising means to intercept acarrier wave and its associated side bands, means to divide the energythus intercepted into two channels, means to reduce the frequency bandwidth of one of said channels, means to substantially eliminatedemodulation in the other of said channels, means to cause thefrequencies in said two channels to beat together, means fordemodulating the product of such beating to produce only the desiredsignal, means to predetermine the phase relation between the twochannels so that the reproduction of signal frequencies is varied fordifferent frequencies in a predetermined manner, and a visual tuningindicator operated from the output of said channel reduced in bandwidth.

6. An apparatus of the class described comprising means to intercept acarrier wave and its associated side bands, means to divide the energythus intercepted into two channels, means to reduce the frequency bandwidth of one of said channels, means to substantially preventdemodulation in the other of said channels, means to cause thefrequencies in said two channels to beat together, means fordemodulating the beat.

,ated from said channel having the reduced frequency band width.

7. An apparatus of the class described comprising means to intercept acarrier Wave and its associated side bands, means to divide the energythus intercepted into two channels, means to maintain the frequency bandwidth of one of said channels sufficiently broad to transmitsubstantially without attenuating the side bands accompanying saidcarrier wave, means to reduce the frequency band width of said otherchannel, means to adjust within predetermined limits the band width ofsaid broad channel, means to cause the frequencies in said two channelsto beat together to produce demodulation, means to translate thefrequencies produced by said demodulation, and means operatedsimultaneously with said band width adjusting means to vary the strengthof the interceptedsignal as the frequency band width of said broadchannel is varied, whereby the amplitude is increased when the frequencyband width is increased and the amplitude is decreased when thefrequency band width is decreased.

8. An apparatus of the class described comprising means to intercept acarrier wave and its associated side bands, means to divide the energythus intercepted into two channels, means to maintain one of saidchannels with a broad enough frequency band to transmit all of the sideband frequencies accompanying said carrier without attenuation, means toreduce the frequency band width of said other channel, means tosubstantially prevent demodulation in said broad channel, means to causethe frequencies in said two channels to beat together, means fordemodulating the beat frequencies so produced to produce only thedesired signal, means to amplify the frequencies produced by suchdemodulation, and means included in said amplifying means to absorb afrequency corresponding to the beat note produced by said carrier and anadjacent unwanted carrier.

9. A radio receiver comprising means to intercept a carrier wave and itsassociated side bands, means to divide the energy thus intercepted intotwo channels, means to maintain the frequency band width of one of saidchannels broad enough so that it will transmit substantially all of theside band frequencies accompanying said carrier wave withoutattenuation, means to reduce the frequency band width of the other ofsaid chan nels without removing all of said side bands, a

balanced demodulator included in said broad channel, means to feed saidother channel into.

said balanced demodulator in such a manner that the frequencies of saidother channel beat with the frequencies introduced to said balanceddemodulator by said broad channel so as to produce demodulation in saidbalanced demodulator, and means to maintain substantially constant thevolume output of said receiver, said means operating from said reducedband width channel and acting upon said receiver at a point precedingsaid energy dividing means.

10. A radio receiver comprising means to intercept a carrier wave andits associated side band frequencies, means to divide the energy thusintercepted into two channels, means to maintain the frequency bandwidth of one of said channels sufficiently broad to pass substantiallyall of said side band frequencies without substantial attenuation, meansto reduce the frequency band width of the other of said channels butstill maintaining some of the side bands, a balanced demodulator in saidbroad channel, means to feed the frequencies of said other channel intosaid balanced demodulator in such a manner that the frequencies of thetwo two channels beat together to produce demodulation, and means topredetermine the phase relation between the two sets of beatingfrequencies so as to vary the strength of different frequencies producedby demodulation in accordance with apredetermined arrangement.

11. A receiver comprising means to intercept a transmitted carrier waveand its associated side bands, a plurality of coupled tuned circuitsseparated by thermionic tubes and connected to said intercepting means,means between the first and last of said coupled circuits to divert someof the energy intercepted by said intercepting means into anotherchannel, means to cause beating between the frequencies of the twochannels thus formed to produce demodulation, and means to vary thefrequency band width of one of said coupled tuned circuits included inboth of said channels and another of said circuits included in only oneof said channels, said circuit included in both of said channels alwayshaving at least as narrow a frequency band width as said circuitincluded in only one of said channels.

ERNEST A. TUBBS.

